32G FOURTH REPORT — 1834. 



the reflexion and refraction of light are ascribed in the theory of 

 emission, it will follow that they must exist in difterent bodies 

 in very different degrees ; so that the amount of bending of the 

 rays, and therefore the position of the diffracted fringes, should 

 vary with the mass, the nature and Xheform of the inflecting 

 body. Now it is clearly ascertained, on the contrary, that alC 

 bodies, whatever be their nature or the form of their edge, pro- 

 duce under the same circumstances fringes identically the same; 

 and in fact the partial interception of light, caused by the inter- 

 position of an obstacle of any kind, seems to be the only condi- 

 tion on which the character of the phenomenon depends. 

 Gravesende seems to have first observed that the nature or den- 

 sity of the body had no effect upon the magnitude of the diffracted 

 fringes ; and the fact has since been confirmed in the fullest 

 manner by almost every inquirer in this branch of experimental 

 science. Oneof the ablest supporters of the theory of emission 

 has admitted that the inflecting forces, if such exist, must be inde- 

 pendent of the chemical nature of the inflecting body, and altoge- 

 ther difterent in their nature from those to which, in the same 

 theory, the phenomena of reflexion and refraction are ascribed*. 

 To ascertain whether the form of the edge had any effect upon 

 the fringes, Fresnel took two plates of steel, the edge of each 

 of which was rounded in one half of its length and sharp in the 

 remaining half, and placed the rounded portion of one edge op- 

 posite the angular part of the other, and vice versa. If, then, the 

 position of the fringes depended on the form of the surface, the 

 effect would thus be doubled, and the fringes appear broken in 

 the middle. They were found, on the contrary, to be perfectly 

 straight throughout their entire lengthf . 



Again, the inflecting forces (though they must be supposed 

 to vary in intensity, with the form and mass of the bod}', and 

 with the distance of the luminous molecule from the edge) can- 

 not be conceived to depend in any way upon the distance pre- 

 viously traversed by the molecule before it arrives in the neigh- 

 bourhood of that edge ; so that the magnitude and position of 



* Biot, Precis elementaire, vol. ii. p. 473, 3""^ Edit. 



•f Memoire sur la Diffraction, p. 370. The Bulletin Z7w»»e««/ for February 

 1828 contains some animadversions on this part of Fresnel's optical labours, in 

 a paper signed by the secretary of the Academy of Sciences of St. Petersburgh, 

 and purporting to be an official reply to some remarks in a former number of 

 the Bulletin on the programme of the prize questions proposed by the Academy. 

 The writers have confounded two experiments of Fresnel which were instituted 

 with different views, and differently reasoned upon. Fresnel's object in this 

 experiment was simply to show that the form of the edge produced no eftect 

 upon the fringes, as it ought to do if diffraction arose from attractive or repul- 

 sive forces extending tosensible distances from bodies. Most of the objections 

 urged in the same paper agaiubt the wave-theory arise, in like niuuuer, in mis- 

 conception. 



